This application is a continuation-in-part of application Ser. No. 429,719, filed Oct. 31, 1989.
This invention relates to pressure-sensitive adhesives for application to substrates or carrier layers to provide laminates having positionable-repositionable properties and to methods of adhering laminates to mounting or application surfaces to provide such properties.
The pressure-sensitive adhesives and laminates of interest herein are characterized by low initial tack and adhesion properties which enable the laminate to be adhered with pressure to a mounting surface and removed therefrom before any appreciable build-up in adhesion occurs. The laminates are initially removable without damage to the substrate or transfer of adhesive to the mounting surface. Preferably, the pressure-sensitive adhesive permits repeated application of the laminate to the surface and removal therefrom during an initial time period of 15 to 30 minutes or longer. Further, the layer of pressure-sensitive adhesive should not stick to itself during such initial time period.
Upon passage of time, the adhesion build-up should be sufficient to secure the laminate to the mounting or application surface in accordance with its intended purpose. For example, permanent bond systems result in a maximum or ultimate adhesion which prevents removal of the laminate from the mounting surface without laminate damage such as tearing or distortion. The development of maximum or ultimate adhesion build-up may take about one week at room temperature.
In many applications, it is preferable that adhesion build-up may also be achieved by application of heat. This enables the development of the ultimate adhesion to be expedited.
The laminates may be used in a wide range decorative or protective applications. For example, the laminates may be used in signs, tapes and vehicle marking such as decorative decals and fleet marking and in architectural applications such as service station canopy decoration. The laminates may be used in the form of tapes, sheets or roll stock. The substrate or carrier layer may comprise a facestock material which can be printed, coated or over-laminated to provide desired indicia.
The term positionable or positionability is used herein to indicate a sufficiently low degree tack to allow a laminate having its adhesive surface in contact with a mounting surface to be slid across the mounting surface without sticking or grabbing. An illustrative test includes the manual sliding of a 3".times.8"laminate test sample along a clean aluminum surface without contact pressure other than the weight of the sample. Positionability is indicated by a smooth sliding movement of the test sample without damage to the specimen or removal of adhesive.
A quantitative test for measurement of pressure-sensitive adhesive tack is set forth in ASTM D 2979-88. Herein, positionability is considered to be achieved by pressure-sensitive adhesives having tack values ranging up to about 90 g. using a Polyken brand tester in accordance with the test procedure.
Adhesion build-up may be quantitatively measured by means of the 180.degree. peel test as set forth in PSTC Test Method No. 1. Herein, 180.degree. peel test values are determined at time intervals ranging from one minute to thirty minutes after the application of the pressure-sensitive adhesive to a mounting surface in order to further characterize tack and adhesion build-up characteristics. Peel values are also measured after one week following the application of the pressure-sensitive adhesive to a surface in order to determine the maximum or ultimate peel strength and degree of adhesion build-up.
Repositionable or repositionability is used herein to indicate the ability to install a laminate with pressure to the mounting or application surface in a final manner and to remove it therefrom repeatedly prior to a build-up of adhesion during an initial time period and without damage to the laminate. Therefore, repositionability is also a function of the strength of the substrate or facestock material. In addition, the laminate also retains its positionability characteristics after removal from the mounting surface. The mounting surface may be a metallic surface, a painted surface or other suitable surface cleaned for adhesive application.
The use of solid particles disposed along the adhesive surface of a pressure-sensitive adhesive layer and partially embedded therein to space the adhesive surface from the mounting or application surface to enable positionability is disclosed in U.S. Pat. No. 4,556,595. A wide variety of particles are disclosed including calcium carbonate, aluminum hydroxide and silica. The particle size is indicated to be less than 10 microns and in the range of 0.001 micron to 3 microns. Upon application of sufficient installation pressure to the laminate, the particles are embedded in the adhesive so as to no longer interfere with the adhesion. Thus, the laminate is not repeatedly positionable.
Japanese Pat. No. 52133339, dated Nov. 8, 1977, is reported to disclose a multi-layered acrylic pressure-sensitive adhesive having silica acid powder of less than 0.1 micron particle size concentrated at the surface thereof. A solvent solution of the adhesive containing dispersed particles is evaporated to form a film which is more than 15 microns thick. It is indicated that release paper is not required, positioning may be done at low pressure and final bonding is achieved at high pressure.
The prior art also discloses a variety of laminate materials having a pressure-sensitive adhesive layer including micro-balloons disposed therein to aid the positionability of the laminate. U.S. Pat. No. 3,331,729 discloses the use of thin-walled fragile micro-balloons which are randomly distributed over and partially embedded in the surface of the adhesive layer. The micro-balloons space the adhesive from the surface to allow positionability of the laminate. Thereafter, sufficient pressure is applied to crush the micro-balloons and permit the adhesive to adhere to the substrate. The laminate is thereby fully installed and not capable of further movement. U.S. Pat. No. 4,376,151 also discloses the use of micro-balloons which allow positionability until a threshold pressure is applied to displace the micro-balloons.
U.S. Pat. No. 3,314,838 discloses a monolayer of micro-balloons covered with a thin film of adhesive which provides a slidable pebbled surface prior to the application of pressure. Upon installation, sufficient pressure is applied to crush the micro-balloons and cause the adhesive to contact the application surface.
The prior art also discloses the addition of particles and micro-balloons to pressure-sensitive adhesives for other purposes. For example, U.S. Pat. No. 4,415,615 discloses the use of thixotropic agents such as fumed silica in its microbubble-filled cellular adhesive layers. Increased cohesiveness is indicated in U.S. Pat. No. 4,710,536 by the addition of hydrophobic silica.